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<a name="boost_explicit_operator_bool_and_boost_constexpr_explicit_operator_bool"></a>BOOST_EXPLICIT_OPERATOR_BOOL and BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL</h2></div>
<div><div class="authorgroup"><div class="author"><h3 class="author">
<span class="firstname">Andrey</span> <span class="surname">Semashev</span>
</h3></div></div></div>
<div><p class="copyright">Copyright &#169; 2013 Andrey Semashev</p></div>
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<a name="boost_explicit_operator_bool_and_boost_constexpr_explicit_operator_bool.legal"></a><p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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<p><b>Table of Contents</b></p>
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<dt><span class="section"><a href="explicit_operator_bool.html#boost_explicit_operator_bool_and_boost_constexpr_explicit_operator_bool.overview">Overview</a></span></dt>
<dt><span class="section"><a href="explicit_operator_bool.html#boost_explicit_operator_bool_and_boost_constexpr_explicit_operator_bool.examples">Examples</a></span></dt>
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<a name="boost_explicit_operator_bool_and_boost_constexpr_explicit_operator_bool.overview"></a><a class="link" href="explicit_operator_bool.html#boost_explicit_operator_bool_and_boost_constexpr_explicit_operator_bool.overview" title="Overview">Overview</a>
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<p>
      <code class="computeroutput"><span class="identifier">BOOST_EXPLICIT_OPERATOR_BOOL</span><span class="special">()</span></code> and <code class="computeroutput"><span class="identifier">BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL</span><span class="special">()</span></code> are compatibility helper macros that expand
      to an explicit conversion operator to <code class="computeroutput"><span class="keyword">bool</span></code>.
      For compilers not supporting explicit conversion operators introduced in C++11
      the macros expand to a conversion operator that implements the <a href="http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Safe_bool" target="_top">safe
      bool idiom</a>. In case if the compiler is not able to handle safe bool
      idiom well the macros expand to a regular conversion operator to <code class="computeroutput"><span class="keyword">bool</span></code>.
    </p>
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<a name="boost_explicit_operator_bool_and_boost_constexpr_explicit_operator_bool.examples"></a><a class="link" href="explicit_operator_bool.html#boost_explicit_operator_bool_and_boost_constexpr_explicit_operator_bool.examples" title="Examples">Examples</a>
</h2></div></div></div>
<p>
      Both macros are intended to be placed within a user's class definition. The
      generated conversion operators will be implemented in terms of <code class="computeroutput"><span class="keyword">operator</span><span class="special">!()</span></code>
      that should be defined by user in this class. In case of <code class="computeroutput"><span class="identifier">BOOST_CONSTEXPR_EXPLICIT_OPERATOR_BOOL</span><span class="special">()</span></code> the generated conversion operator will be
      declared <code class="computeroutput"><span class="keyword">constexpr</span></code> which requires
      the corresponding <code class="computeroutput"><span class="keyword">operator</span><span class="special">!()</span></code>
      to also be <code class="computeroutput"><span class="keyword">constexpr</span></code>.
    </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T</span> <span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">my_ptr</span>
<span class="special">{</span>
    <span class="identifier">T</span><span class="special">*</span> <span class="identifier">m_p</span><span class="special">;</span>

<span class="keyword">public</span><span class="special">:</span>
    <span class="identifier">BOOST_EXPLICIT_OPERATOR_BOOL</span><span class="special">()</span>

    <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="special">!</span><span class="identifier">m_p</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
      Now <code class="computeroutput"><span class="identifier">my_ptr</span></code> can be used in conditional
      expressions, similarly to a regular pointer:
    </p>
<pre class="programlisting"><span class="identifier">my_ptr</span><span class="special">&lt;</span> <span class="keyword">int</span> <span class="special">&gt;</span> <span class="identifier">p</span><span class="special">;</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">p</span><span class="special">)</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"true"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
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<a name="boost_explicit_operator_bool_and_boost_constexpr_explicit_operator_bool.history.h0"></a>
      <span class="phrase"><a name="boost_explicit_operator_bool_and_boost_constexpr_explicit_operator_bool.history.boost_1_55"></a></span><a class="link" href="explicit_operator_bool.html#boost_explicit_operator_bool_and_boost_constexpr_explicit_operator_bool.history.boost_1_55">boost
      1.55</a>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          The macro was extracted from Boost.Log.
        </li></ul></div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"><p><small>Last revised: November 17, 2013 at 17:08:52 GMT</small></p></td>
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